Heat Exchanger and Header for the Same

ABSTRACT

A header for a heat exchanger includes a first and a second cylindrical fluid manifold extending in parallel. Each of the first and second manifolds have tube slots that extend through an arcuate wall section of the manifold. A thickened wall section of the header having a generally triangular wall section is bounded by the first and second fluid manifolds and by a planar outer surface of the header. An aperture extends through the thickened wall section to provide a fluid communication pathway between the first and second cylindrical fluid manifolds.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 62/353,618, filed Jun. 23, 2016, the entire contents of which arehereby incorporated by reference.

BACKGROUND

Heat exchangers are used to transfer thermal energy from one stream offluid at a first, higher temperature to another stream of fluid at asecond, lower temperature. Oftentimes such heat exchangers are used toremove waste heat from a process fluid such as oil, coolant, or the likeby transferring that heat to a flow of cooler air directed to passthrough the heat exchanger.

In certain applications, the process fluid to be cooled is also at anoperating pressure that is substantially greater than the ambientatmospheric pressure of the heat exchanger's surroundings. As a result,it becomes necessary for the heat exchanger to be designed to withstandthe pressure forces that result from the process fluid passing throughthe heat exchanger. This can become challenging, especially in caseswhere the heat exchanger is to be used in large systems and machinerysuch as, for example, construction equipment, agricultural machines, andthe like. As the size of the machine or system increases, the flow rateof the process fluid also increases, necessitating larger heatexchangers to accommodate both the heat transfer requirements and thefluid flow rates.

In some particular styles of heat exchangers, the fluid to be cooled isdirected through an array of flat tubes extending between two tanks orheaders. As such heat exchangers become larger, they can havesubstantially large surface areas exposed to the pressure of the processfluid, especially in the tank or header areas, and the force of thefluid pressure acting on these large surfaces can lead to destructivemechanical stresses in the heat exchanger structure. The ability towithstand such pressures can be improved through the use of circularheader profiles, but circular headers can be difficult to package withina compact space as the required size of the heat exchanger increases.

SUMMARY

According to an embodiment of the invention, a header for a heatexchanger includes a first and a second cylindrical fluid manifoldextending in parallel. Each of the first and second manifolds have tubeslots that extend through an arcuate wall section of the manifold. Athickened wall section of the header having a generally triangular wallsection is bounded by the first and second fluid manifolds and by aplanar outer surface of the header. An aperture extends through thethickened wall section to provide a fluid communication pathway betweenthe first and second cylindrical fluid manifolds.

In some embodiments, the header includes a plug that is inserted into anopening that extends through the planar outer surface to the aperture.In some such embodiments the plug is brazed to the planar outer surface.In some embodiments the plug includes an integral mounting pin thatextends outwardly from the header in a direction perpendicular to theplanar outer surface. In some embodiments the arcuate wall section ofone of the manifolds defines a minimum wall thickness of the header, andthe insertion depth of the plug through the opening is approximatelyequal to that minimum wall thickness.

In some embodiments the header includes a third cylindrical fluidmanifold adjacent to and parallel to the second fluid manifold. A secondthickened wall section of the header having a generally triangular wallsection is bounded by the third and second fluid manifolds and by theplanar outer surface of the header. In some such embodiments an apertureextends through the second thickened wall section to provide a fluidcommunication pathway between the second and third fluid manifolds.

In some embodiments the header includes a first and a second mountingflange extending from the header. The first mounting flange defines afirst mounting plane and the second mounting flange defines a secondmounting plane, with both the first and second mounting planes beingoriented parallel to one another and perpendicular to the planar outersurface of the header. A first mounting hole extends through the firstmounting flange and is aligned with a second mounting hole that extendsthrough the second mounting flange. In some such embodiments all of thefluid manifolds are entirely located between the first and secondmounting planes.

According to another embodiment, a method of making a header for a heatexchanger includes providing an extruded section with two unconnectedcylindrical volumes arranged therein and with a planar outer surface,and machining through the planar outer surface to define an aperturebetween the two cylindrical volumes. The act of machining through theplanar outer surface creates an opening in that surface, and a plug isinserted into the opening. In some embodiments the plug is brazed to theextruded section in order to secure it within the opening. In someembodiments a series of tube slots are formed into arcuate wall sectionsof the two cylindrical volumes opposite the planar outer surface.

In some embodiments of the invention, a heat exchanger includes a firstseries of parallel-arranged flat tubes to convey a first fluid throughthe heat exchanger, and a second series of parallel-arranged flat tubesto convey a second fluid through the heat exchanger. The first andsecond fluids are, for example, two fluids used within a powergenerating system (such as an internal combustion engine system, forexample) that are cooled within the heat exchanger by a flow of airpassing through the heat exchanger over the outer surfaces of the flattubes. Such fluids can include, but are not limited to, liquid coolants,water, refrigerant, oil, and others.

The first and second series of flat tubes extend between a first and asecond header arranged at opposing ends of the heat exchanger. Each ofthe headers has at least two cylindrical fluid manifolds, each of whichis provided with tube slots that extend through an arcuate wall sectionof the header. Ends of the tubes are received within the tube slots.

In some embodiments, the headers are provided with more than twocylindrical fluid manifolds and the heat exchanger includes acorresponding additional series of parallel-arranged flat tubes. In somesuch embodiments, the additional series of tubes carries a third fluidthat is distinct from the first and the second fluids. By distinct ismeant that there is no hydraulic connection within the heat exchangerbetween the fluids. The fluids could, however, be hydraulicallyconnected elsewhere within the system. In other embodiments, however, athird series of flat tubes is hydraulically connected within the heatexchanger to one of the first and second series of flat tubes, so thatthe corresponding one of the first and second fluids also flows throughthat third series of flat tubes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a heat exchanger including headersaccording to an embodiment of the invention.

FIG. 2 is a partial perspective view of a section of a heat exchangercore used in the heat exchanger of FIG. 1.

FIG. 3 is an exploded perspective view of one of the headers of FIG. 1.

FIG. 4 is a plan cross-sectional view of the header of FIG. 3.

FIG. 5 is a plan cross-sectional view of a component of the header ofFIG. 3.

FIG. 6 is a partial perspective view of a heat exchanger includingheaders according to another embodiment of the invention.

FIG. 7 is a partial plan view of the heat exchanger of FIG. 6.

FIG. 8 is a partial plan view of a component of one of the headers ofFIG. 6.

FIG. 9 is a plan view showing two separate fluid circuits within a heatexchanger according to some embodiments of the invention.

FIG. 10 is a plan view showing three separate fluid circuits within aheat exchanger according to some embodiments of the invention.

FIG. 11 is another plan view showing two separate fluid circuits withina heat exchanger according to some embodiments of the invention.

FIG. 12 is another plan view showing two separate fluid circuits withina heat exchanger according to some embodiments of the invention.

FIG. 13 is a front view showing two separate fluid circuits within aheat exchanger according to some embodiments of the invention.

FIG. 14 is a front view showing a single fluid circuit within a heatexchanger according to some embodiments of the invention.

DETAILED DESCRIPTION

Before any embodiments of the invention are explained in detail, it isto be understood that the invention is not limited in its application tothe details of construction and the arrangement of components set forthin the following description or illustrated in the accompanyingdrawings. The invention is capable of other embodiments and of beingpracticed or of being carried out in various ways. Also, it is to beunderstood that the phraseology and terminology used herein is for thepurpose of description and should not be regarded as limiting. The useof “including,” “comprising,” or “having” and variations thereof hereinis meant to encompass the items listed thereafter and equivalentsthereof as well as additional items. Unless specified or limitedotherwise, the terms “mounted,” “connected,” “supported,” and “coupled”and variations thereof are used broadly and encompass both direct andindirect mountings, connections, supports, and couplings. Further,“connected” and “coupled” are not restricted to physical or mechanicalconnections or couplings.

A heat exchanger 1 according to an embodiment of the invention isdepicted in FIG. 1, and includes a heat exchange core 2 bounded betweentwo side plates 4. The heat exchange core 2 is constructed as a stackedand brazed assembly of alternating layers of flat tubes 5 and corrugatedfins 6, as shown in the core detail of FIG. 2. The tubes 5 and fins 6are preferably formed of an aluminum alloy so that the heat exchanger 1can be built to be lightweight and highly efficient in the transfer ofheat between a first fluid flowing through the interiors of the tubes 5and a second fluid (air, for example) passing through the corrugationsof the fins 6. Such a heat exchanger 1 can be used as, for example, avehicular powertrain cooling heat exchanger to cool engine oil,transmission oil, engine coolant, or some other fluid from whichdissipation of heat is desired.

Open ends of the tubes 5 are received into headers 3 arranged atopposing ends of the heat exchanger 1. Each header 3 is an assembly ofparts, shown in exploded view in FIG. 3. The header 3 includes anextruded section 7 that extends over generally the full stacked heightof the heat exchange core 2, and provides a number of cylindrical fluidmanifolds 8 that distribute the first fluid to, or receive the firstfluid from, the array of tubes 5. The number of cylindrical fluidmanifolds 8 that is provided by each extruded section 7 corresponds tothe number of tubes 5 provided in each row of tubes of the core 2 (e.g.two, in the exemplary embodiment of FIGS. 1-5).

As best seen in the cross-sectional view of FIG. 5, each of thecylindrical fluid manifolds 8 is bounded by an arcuate wall section 9over a majority of the circular periphery of the manifold, with thatarcuate wall section 9 having a generally constant wall thickness(indicated by the reference number 20). On the core-facing side ofheader 3, the arcuate wall sections 9 of the two adjacent manifolds 8merge together. On the opposing (i.e. the non-core-facing) side of theheader 3 a planar outer surface 14 of the header is provided. The planarouter surface 14, together with the cylindrical manifolds 8, bounds athickened wall section 21 of the extruded section 7. The thickened wallsection 21 has a generally triangular cross-section, as indicated inFIG. 5 by the dashed triangle 22), with a wall thickness that issubstantially greater than the wall thickness 20 of the arcuate wallsections 9. As indicated by FIG. 5, the cross-section of the thickenedwall section 21 can deviate somewhat from a truly triangular shape whilestill exhibiting a generally triangular cross-section.

Tube slots 13 are provided along the lengths of the headers 3 to receivethe ends of the tubes 5 into the corresponding cylindrical fluidmanifolds 8. The tube slots 13 can be formed into the extruded section 7by, for example, saw-cutting or piercing. Each of the tube slots 13extends through one of the arcuate wall sections 9, and has a width andheight that generally corresponds to the major and minor dimensions ofthe flat tubes 5. The ends of the flat tubes 5 are preferably insertedinto the tube slots 13 after the flat tubes 5 and the fins 6 have beenstacked to form the core 2, so that the tubes 5 can be brazed to theheaders 3 in the same brazing operation as is used to join the flattubes 5 to the fins 6, thereby creating leak-free joints at thetube-to-header interfaces.

The cylindrical fluid manifolds 8 are hydraulically connected by way ofone or more apertures 15 that extend through the thickened wall section21 at one or more locations along the length of the header 3. Such anaperture 15 can be formed by a machining operation such as drilling ormilling through the planar surface 14 to a predetermined depth, in whichcase the forming of the aperture 15 can define a circular opening 40 inthe planar surface 14, as shown in FIG. 3. The predetermined depth isselected to be less than the depth that would be required in order toremove all of the material separating the cylindrical fluid manifolds 8at that location. As best seen in the cross-sectional view of FIG. 4,the aperture 15 of the exemplary embodiment has the material in thethickened wall section 21 removed to a depth, as measured from theplanar surface 14, that is approximately equal to the radius of thearcuate wall sections 9. While the exemplary embodiment depicts acircular opening 40 formed in the planar outer surface 14, it should beunderstood that other machining methods might result in non-circularopenings.

A plug 12 can be inserted into the opening 40 defined by the forming ofthe aperture 15 at the planar outer surface 14 in order to provide afluid-tight seal between the fluid manifolds 8 and the outsideenvironment external to the header 3. The plug 12 includes an insertionportion 18 with a profile that generally matches the opening 40 createdin the planar surface 14, so that the plug 12 can be partially insertedinto that opening 40 with minimal clearance between side surfaces of theinsertion portion 18 and the opening 40. A peripheral flange portion 17extends beyond the outer periphery of the insertion portion 18 by anamount sufficient to engage and bear upon the planar surface 14surrounding the opening 40, thereby limiting the insertion depth of theplug 12. In some especially preferable embodiments, such as theexemplary embodiment of FIG. 4, the height of the insertion portion 18(and, therefore, the depth of insertion of the plug 12 into the opening40) is approximately equal to the wall thickness 20 of the arcuate wallsections 9.

A groove 25 can be provided in the face of the peripheral flange portion17 that is disposed against the planar surface 14, and can be used toaccommodate a ring of braze material 16. The plug 12, along with thering of braze material 16, can be assembled to the extruded section 7prior to brazing of the heat exchanger 1, so that the plug 12 can besecured into the header 3 during the brazing operation. In someembodiments it may be more preferable to instead use a braze foil, brazepaste, or clad braze layer on either the plug 12 or the extruded section7, in which case the braze ring 16 and the groove 25 may be eliminated.

One or more of the plugs 12 can be provided with an integral mountingpin 19 extending outwardly away from the header in a directionperpendicular to the planar outer surface 14. The integral mounting pins19 can be accommodated into corresponding holes of other components towhich the heat exchanger 1 is to be assembled in order to, for example,secure the heat exchanger 1 within a cooling module. Annular vibrationisolators can be conveniently assembled over the mounting pin 19 andbear against the peripheral flange portion 17 of the plug 12.

At the ends of the header 3, the cylindrical fluid conduits 8 are sealedwith either end caps 11 or fluid ports 10. In the exemplary embodimentof FIG. 1, each of the headers 3 is provided with three end caps 11 andone fluid port 10, so that the fluid to be cooled within the heatexchanger 1 can be received into one of the headers 3 (the inlet header)through its fluid port 10 and can be removed from the other one of theheaders 3 (the outlet header) through its fluid port 10. Although thefluid is directly received into only one of the cylindrical fluidmanifolds 8 of the inlet header 3, the apertures 15 provided in thatheader 3 allow at least some of the fluid to pass into the adjacentcylindrical fluid manifold 8 so that the flat tubes 5 connected to eachof those fluid manifolds 8 are placed hydraulically in parallel with oneanother. In similar fashion, the fluid received into that one of thecylindrical fluid manifolds 8 of the outlet header 3 can be transferredby way of the apertures 15 into the cylindrical fluid manifold 8 havingthe outlet port 10.

By placing multiple fluid manifolds 8 in hydraulic parallel, the presentinvention is able to provide a more robust design for applicationswherein the fluid to be cooled is at an elevated pressure. The abilityof the fluid manifold to withstand the elevated internal pressuresimposed by the fluid is increased by reducing the diameter of each fluidmanifold, without sacrificing the total flow area provided by the flattubes 5. To that end, it should be understood that the number ofcylindrical fluid manifolds 8 that may be provided in each of theheaders 3 is not limited to two. Additional fluid manifolds 8 can beprovided, and can be fluidly connected to adjacent fluid manifoldsthrough additional apertures 15. It should be understood that amulti-pass heat exchanger can also be provided by placing apertures 15between some, but not all, of the adjacent fluid manifolds 8.

As one non-limiting example of a heat exchanger having more than twocylindrical fluid manifolds within the headers, a portion of a heatexchanger 1′ is depicted in FIGS. 6-7. The heat exchanger 1′ includes aheat exchange core 2′ that is substantially similar to the heat exchangecore 2 depicted in FIG. 2, except that three rows of flat tubes 5 areprovided in each layer of tubes. Similarly, a header 3′ provided ateither end of the core 2′ (only a single header is shown) includes anextruded section 7′ that includes three cylindrical fluid manifolds 8arranged side-by-side to receive the ends of the tubes 5 in similarfashion as was described previously with reference to the embodiment ofFIG. 1.

The extruded section 7′, shown in greater detail in FIG. 8, is similarto the previously described extruded section 7 in that it includes anarcuate wall section 9 over a majority of the circular periphery of eachmanifold 8, with the arcuate wall sections 9 having a generally constantwall thickness. The ends of the tubes 5 are received into the fluidmanifold through slots provided in those arcuate wall sections 9. Aplanar outer surface 14 is again provided on the opposing (i.e. thenon-core-facing) side of the header 3′.

The extruded header section 7′ can optionally be provided with mountingflanges 33, as shown in FIGS. 6-8. The mounting flanges 33 extend in adirection that is perpendicular to the planar surface 14 and is directedaway from the heat exchange core 2, thereby defining a pair of mountingplanes 35 (i.e. a first mounting plane 35 and a second mounting plane35) for the heat exchanger 1′ which are likewise arranged perpendicularto the planar surface 14. In some preferred embodiments the cylindricalfluid manifolds 8 provided by the header 3′ are all located entirelybetween the pair of mounting planes 35. While the mounting flanges 33are depicted as extending from the arcuate wall sections 9, it should beunderstood that they can alternatively or in addition extend from theplanar surface 14.

The mounting flanges 33 can be used to structurally mount the heatexchanger 1′ into a cooling module or other assembly, as shown in FIGS.6-7. A U-channel 23 that forms part of the cooling module or otherassembly includes parallel, spaced-apart legs 27 joined by a connectingsection 26, with the space between the legs 27 sized to be sufficientlylarge to allow for the header 3′ to be received there between.Connection assemblies 24 structurally connect the header 3′ to theU-channel 23, and include compressible rubber isolators 32 that areinserted into holes placed within the U-channel 23 so that a portion ofeach isolator 32 is arranged inside of the U-channel 23 and anotherportion of the isolator 32 is arranged outside of the U-channel 23. Theisolators 32 are provided in pairs at locations that align withcorresponding mounting holes 34 provided in the flanges 33, so that abolt 28 or other similar fastener can be inserted through the pairedisolators 32 and the corresponding mounting holes 34 in the flanges 33.Washers 31 are provided between a head 29 of the bolt 28 and one of thepaired isolators and between a nut 30 that is threaded onto the end ofthe bolt 28 and the other one of the paired isolators. Each of theconnection assemblies thus includes a bolt 28, nut 30, pair of washers31, and pair of isolators 32. By tightening the nuts 30 of theconnection assemblies 24, the heat exchanger 1′ can be secured to theU-channel 23. It should be understood that the connection assemblies 24can be used either as an alternative to, or in addition to, the mountingpins 19 that were previously described.

In some embodiments, such as the ones shown in FIGS. 9-12, it may bedesirable to incorporate fluid circuits for multiple fluids to be cooledwithin a single heat exchanger 1, 1′. In such an embodiment, a firstseries 41 of the flat tubes 5 is used to convey a first one of thefluids to be cooled, while a second series 42 of the flat tubes 5 isused to convey a second one of the fluids to be cooled. As shown in FIG.9, each of the headers 3 arranged at opposing ends of the heat exchanger1 again includes a pair of cylindrical fluid manifolds 8. However, thefluid manifolds 8 within a header 3 can be hydraulically isolated fromone another, thereby providing a fluid manifold for the first fluid ineach header 3 (the manifolds 43, 45) and additionally a fluid manifoldfor the second fluid in each header 3 (the manifolds 44, 46).

The number of separate fluid flow circuits provided in a given heatexchanger is not limited. Additional rows of tubes can be added, such asby using the previously described headers 3′ having three cylindricalfluid manifolds 8 to provide a three-fluid heat exchanger 1′, as shownin FIG. 10. Such a heat exchanger includes a third series 47 of the flattubes 5, which extends between additional fluid manifolds 48, 49provided within the headers 3′. Incorporating multiple heat exchangefunctions within a single heat exchanger in this fashion can provideadvantages by reducing the space requirements, simplifying systemintegration and assembly, and reducing overall cost, among others. Itshould be understood that the invention is not limited to the number offluids and/or series of tubes shown, and that the number can be furtherincreased in a similar manner to that shown.

As shown in FIGS. 11 and 12, the number of tube rows or series can alsobe increased without increasing the number of discrete fluids beingcooled within the heat exchanger. In the embodiment of FIG. 11, theadditional series 47 is used to provide additional flat tubes 8 forconveying the first fluid through the heat exchanger. The previouslydescribed apertures 15 are provided between the cylindrical manifolds43, 48 in one of the headers 3′ and between the cylindrical manifolds45, 49 in the other header 3′ in order to place the series 41 of flattubes 5 fluidly in parallel with the series 47 of flat tubes 5. Such anarrangement can be especially desirable when the flow area required forthe first fluid is substantially greater than the flow area required forthe second fluid.

As shown in FIG. 12, in some cases it may be more desirable to providethe apertures 15 in only one of the headers 3, thereby placing theseries 41 of flat tubes 5 fluidly in series with the series 47 of flattubes 5. As indicated by the arrow in FIG. 12, the first fluid is firstreceived into the cylindrical fluid manifold 48, passes through theseries 47 of tubes to the cylindrical fluid manifold 49, transfers byway of one or more apertures 15 to the cylindrical fluid manifold 45,and passes through the series 41 of tubes to the cylindrical fluidmanifold 43. More effective heat transfer between the air passingthrough the heat exchanger and the first fluid can be achieved with suchan arrangement.

Additional flow circuiting options can also be achieved through the useof baffles 53 inserted into one or both headers 3, 3′, as shown in FIGS.13 and 14. Such a fluid baffle 53 can serve to separate a singlecylindrical fluid manifold 8 into two separate fluid manifolds (e.g. 43and 51, 45 and 52), as depicted. A series 41 of flat tubes 5 and aseries 50 of flat tubes 5 are thereby arranged within the same row oftubes of the heat exchanger. Such an arrangement can provide yet anotheralternative method by which multiple fluids can be accommodated within asingle heat exchanger in an optimized fashion when the flow arearequirements for each of the fluids is vastly different. Specifically,the location of the baffles 53 along the length of the headers 3, 3′ canbe selected so that the total number of tubes 5 within the row isappropriately divided between the series 41 and the series 50.Additionally, one or more additional rows of flat tubes can be providedfor one or more additional fluids, as was described with reference toFIGS. 9 and 10.

In addition, the flow circuiting of FIG. 9, FIG. 11, and FIG. 13 can becombined within a single heat exchanger 1 to provide even greaterflexibility. By way of example, such a heat exchanger can have two ormore sets of tubes such as the sets 41 and 42, arranged in rows withtube ends of each set extending through an arcuate wall of a header 3arranged at each end of the heat exchanger to communicate withcylindrical fluid manifolds 8. A third set of tubes can be arranged in acommon row with one of the first two sets of tubes, such as is shown inFIG. 13 for the sets 50 and 41. Baffles 53 are placed in the headers toseparate the cylindrical chambers corresponding to those two sets oftubes, in order to provide hydraulic isolation between them. Themanifolds for one of those sets of tubes (e.g. the manifolds 51, 52 forthe set 50) can be fluidly coupled to the cylindrical manifolds for anadjacent row of tubes by apertures 15, so that the same fluid flows inparallel through two different sets of tubes, with one the sets of tubesbeing arranged in a common row as another set of tubes for anotherfluid. In this manner, one of the fluids can be transported through theheat exchanger by all of the flat tubes 5 of one of the sets of tubes,and by some, but not all, of the flat tubes 5 of another one of the setsof tubes. Such an arrangement can be especially desirable when thenumber of tubes necessary (for example, due to heat transferconsiderations or pressure drop considerations or both) for one of thefluids is substantially greater than the number of tubes necessary foranother one of the fluids.

It should be understood that the various embodiments depicted in FIGS.9-14 are depicted in simplified form, and that structural details of theheat exchangers that have been excluded from those figures may still bepresent. Particularly, it should be understood that any or all of thepreviously described features, including the plugs 12, mounting pins 19,and mounting flanges 33, can be included in any of those embodiments.

Various alternatives to the certain features and elements of the presentinvention are described with reference to specific embodiments of thepresent invention. With the exception of features, elements, and mannersof operation that are mutually exclusive of or are inconsistent witheach embodiment described above, it should be noted that the alternativefeatures, elements, and manners of operation described with reference toone particular embodiment are applicable to the other embodiments.

The embodiments described above and illustrated in the figures arepresented by way of example only and are not intended as a limitationupon the concepts and principles of the present invention. As such, itwill be appreciated by one having ordinary skill in the art that variouschanges in the elements and their configuration and arrangement arepossible without departing from the spirit and scope of the presentinvention.

What is claimed is:
 1. A heat exchanger comprising: a first plurality offlat tubes to convey a first fluid through the heat exchanger; a secondplurality of flat tubes to convey a second fluid through the heatexchanger; a first header arranged at an end of the heat exchanger, thefirst header including a first cylindrical fluid manifold having aplurality of tube slots extending through a first arcuate wall sectionof the first header, a second cylindrical fluid manifold extendingparallel and adjacent to the first cylindrical fluid manifold having aplurality of tube slots extending through a second arcuate wall sectionof the first header, and a thickened wall section bounded by the firstcylindrical fluid manifold, the second cylindrical fluid manifold, and aplanar outer surface of the first header, the thickened wall sectionhaving a generally triangular cross-section; and a second headerarranged at an opposing end of the heat exchanger, the second headerincluding a third cylindrical fluid manifold having a plurality of tubeslots extending through a first arcuate wall section of the secondheader, a fourth cylindrical fluid manifold extending parallel andadjacent to the third cylindrical fluid manifold having a plurality oftube slots extending through a second arcuate wall section of the secondheader, and a thickened wall section bounded by the third cylindricalfluid manifold, the fourth cylindrical fluid manifold, and a planarouter surface of the second header, the thickened wall section having agenerally triangular cross-section; wherein tube ends of the firstplurality of tubes are received through the tube slots extending throughthe first arcuate wall sections of both the first and second header, andwherein tube ends of the second plurality of flat tubes are receivedthrough the second arcuate wall sections of both the first and secondheaders.
 2. The heat exchanger of claim 1, further comprising a thirdplurality of flat tubes to convey the first fluid through the heatexchanger, the first header additionally including a fifth cylindricalfluid manifold extending parallel and adjacent to the first cylindricalfluid manifold having a plurality of tube slots extending through athird arcuate wall section of the first header, and the second headeradditionally including a sixth cylindrical fluid manifold extendingparallel and adjacent to the third cylindrical fluid manifold having aplurality of tube slots extending through a third arcuate wall sectionof the second header, the first header having another thickened wallsection with a generally triangular cross-section bounded by the firstfluid manifold, the fifth fluid manifold, and the planar outer surfaceof the first header, and the second header having another thickened wallsection with a generally triangular cross-section bounded by the thirdfluid manifold, the sixth fluid manifold, and the planar outer surfaceof the second header.
 3. The heat exchanger of claim 2, wherein thefirst and the third pluralities of tubes define two sequentiallyarranged flow passages through the heat exchanger for the first fluid.4. The heat exchanger of claim 2, wherein the second header includes anaperture extending through the thickened wall section between the thirdfluid manifold and the sixth fluid manifold in order to fluidly couplethe first and the third pluralities of tubes.
 5. The heat exchanger ofclaim 4, wherein the first header includes an aperture extending throughthe thickened wall section between the first fluid manifold and thefifth fluid manifold in order to fluidly couple the first and the thirdpluralities of tubes.
 6. The heat exchanger of claim 2, wherein thetubes of the first plurality of tubes are hydraulically in parallel withthe tubes of the third plurality of tubes.
 7. The heat exchanger ofclaim 1, further comprising a third plurality of flat tubes to convey afluid through the heat exchanger, the first header additionallyincluding a fifth cylindrical fluid manifold extending concentricallywith the first cylindrical fluid manifold having a plurality of tubeslots extending through the first arcuate wall section of the firstheader, and the second header additionally including a sixth cylindricalfluid manifold extending concentrically with the third cylindrical fluidmanifold having a plurality of tube slots extending through the firstarcuate wall section of the second header, the first header havinganother thickened wall section with a generally triangular cross-sectionbounded by the fifth fluid manifold, the second fluid manifold, and theplanar outer surface of the first header, and the second header havinganother thickened wall section with a generally triangular cross-sectionbounded by the sixth fluid manifold, the fourth fluid manifold, and theplanar outer surface of the second header.
 8. The heat exchanger ofclaim 7, wherein the third fluid manifold and the sixth fluid manifoldare in direct fluid communication with one another so that the firstfluid flows sequentially through the first and the third pluralities oftubes.
 9. The heat exchanger of claim 7, wherein the second headerincludes an aperture extending through the thickened wall sectionbetween the fourth fluid manifold and the sixth fluid manifold in orderto fluidly couple the second and the third pluralities of tubes.
 10. Theheat exchanger of claim 9, further comprising a plug inserted into anopening extending through the planar outer surface to the aperture. 11.The heat exchanger of claim 9, wherein the first header includes anaperture extending through the thickened wall section between the secondfluid manifold and the fifth fluid manifold in order to fluidly couplethe second and the third pluralities of tubes.
 12. The heat exchanger ofclaim 11, further comprising a first plug inserted into an openingextending through the planar outer surface of the first header to theaperture in the first header, and a second plug inserted into an openingextending through the planar outer surface of the second header to theaperture in the second header.
 13. A header for a heat exchanger,comprising: a plurality of parallel arranged cylindrical fluidmanifolds; a plurality of arcuate wall sections having a constant wallthickness, each of the arcuate wall sections corresponding to one of theplurality of cylindrical fluid manifolds and each having a plurality oftube slots extending through the constant wall thickness to thecorresponding fluid manifold; and one or more thickened wall sectionsbounded by two adjacent ones of the plurality of cylindrical fluidmanifolds and a planar outer surface of the header, the one or morethickened wall sections each having a generally triangularcross-section.
 14. The header of claim 13, further comprising one ormore apertures extending through the one or more thickened wall sectionsto provide a fluid communication pathway between those cylindrical fluidmanifolds bounding the one or more thickened wall sections.
 15. Theheader of claim 14, further comprising one or more plugs in one-to-onecorrespondence with the one or more apertures, each plug inserted intoan opening extending through the planar outer surface to thecorresponding aperture.
 16. The header of claim 15, wherein a portion ofeach of the one or more plugs is brazed to the outer planar surface. 17.The header of claim 15, wherein at least some of said plugs includes amounting pin integral with the plug and extending outwardly from theheader in a direction perpendicular to the planar outer surface.
 18. Theheader of claim 15, wherein an insertion depth of each of the one ormore plugs is equal to the constant wall thickness of the arcuate wallsections.
 19. The header of claim 13, further comprising: a firstmounting flange extending from the header and defining a first mountingplane; a first mounting hole extending through the first mountingflange; a second mounting flange extending from the header and defininga second mounting plane parallel to the first mounting plane; and asecond mounting hole extending through the second mounting flange andaligned with the first mounting hole, wherein the first and secondmounting planes are oriented perpendicular to the planar outer surfaceof the header.
 20. The header of claim 19, wherein the plurality ofparallel arranged cylindrical fluid manifolds is entirely locatedbetween the first and second mounting planes.